Various systems have been devised and implemented to control the change of gear ratios in an automatic transmission. This is generally done with a hydraulic system, including a hydro-mechanical control valve mechanism for modifying the pressure applied to various friction elements in the transmission and to selectively hold and release different components of a planetary gear set.
A great deal of research has been directed to optimizing the shift "quality". In general, it is not desirable to have a shift of a very brief duration, as this produces a jerk by the rapid change in vehicle acceleration, which is very noticeable and found objectionable by most drivers. If the shift time is stretched out for too long a period, undue wear is imposed on the friction elements and other components of the transmission. Therefore, the optimum shift quality occurs somewhere between the too-short time duration, producing the objectionable jerk sensation, and the too-long time duration producing the objectionable component wear. A comprehensive treatment of this subject has been provided by F. J. Winchell and W. D. Route in "Ratio Changing the Passenger Car Automatic Transmission", which appears as Chapter 10 in the SAE publication "Design Practices -- Passenger Car Automatic Transmissions", Copyright by the Society of Automative Engineers, Inc., 1973. In particular FIG. 21 of this chapter depicts speed, torque and pressure variations during a power-on upshift.
A significant advance in ratio-changing control has been achieved in a control system which regulates the energization of the on-coming friction element, in which the off-going friction element is embodied in a one-way clutch which automatically releases. Hence, the control is directed only to the precise regulation of the on-coming element in a three member planetary gear set. This significant advance is described and claimed in the earlier application of Alan J. Miller and John S. Ivey entitled "Transmission Control System", filed Feb. 27, 1976, Ser. No. 661,896, now Pat. No. 4,031,782 and which is assigned to the assignee of this invention.
Such earlier systems generally use a controlled friction element in parallel with the one-way clutch for reasons explained therein. Properly timing a shift wherein one friction element is released and another is engaged is more difficult and particularly where a downshift is involved and establishment of the lower ratio before the engine RPM has increased to the proper level will produce a faulty shift. It is therefore a principal object of this invention to provide an electronic control system for down-shifting an automatic transmission, in which a simplified and precise control is provided for the on-coming friction element and for the off-going friction element.